1. Field of the Invention
The present invention relates to generally to self-healing communications networks, and more specifically to a ring topology network which supports multiplex signals on protection communication paths during failures by fast switching from working communication paths.
2. Description of the Related Art
Ring topology networks, particularly, optical ring networks are currently receiving attention because of the number of wavelengths that can be multiplexed onto a single optical link is increasing due to recent innovative techniques. A number of technical publications deal with this topic. A four-fiber ring network is discussed in a technical paper xe2x80x9cMultiwavelength Survivable Ring Network Architecturesxe2x80x9d, A. F. Elrefaie, Proceedings of ICC ""93, pages 1245-1251, 1993. According to this publication, a loopback fault recovery method is described. In a four-fiber ring network where optical links are interconnected by a number of network nodes so that working rings are formed for transmission of signals in opposite directions of the ring topology and protection rings are formed for transmission of signals in opposite directions of the ring topology. The protection rings respectively correspond to the working rings and the direction of transmission of each protection ring is also opposite to the direction of transmission of the corresponding working ring. Optical paths are established on each of the working and protection rings between network nodes. If a working optical path between source and destination nodes fails, two loopback points are formed, one on each end of the affected link of the working path, for connecting ends of the corresponding protection optical path to unaffected sections of the working path so that a recovery route is established between the source and destination nodes.
Since the loopback points are close to the location of the failure, the recovery route can be quickly established by nodes adjacent to the fault location and there is no need to exchange fault recovery message between nodes involved. However, the length of the recovery route is significantly long. If a working path spans across one half of its ring, the length of the recovery route would become one and half times the whole length of the ring.
A two fiber ring network is described in a technical paper xe2x80x9cAn Optical FDM-Based Self-Healing Ring Network Employing Arrayed Waveguide Grating Filters and EDTA""s with Level Equalizersxe2x80x9d, Hiromu Toba et al., IEEE Journal on Selected Areas in Communications, Vol. 14, No. 5, pages 800-813. In the two fiber ring network, one of the two rings is used as a working ring for transmission of signals in one direction of the ring topology and the other for transmission of the same signals in the opposite direction. A working path is established on the working ring between two nodes and a corresponding protection path is established between them on the protection ring. Under normal conditions, signals from the source node are forwarded onto the working path as well as onto the protection path. If the working path fails instant switching occurs at these two nodes to continue the communication over the protection path.
Although all signals can be fully and quickly recovered on the protection path, the constant use of the protection path, utilization efficiency of the transmission mediums is low.
It is therefore an object of the present invention to provide a ring topology network which requires short-length fault recovery routes and ensures high efficient utilization of transmission mediums.
According to a first aspect of the present invention, there is provided a communications network comprising a plurality of transmission links and a plurality of nodes for interconnecting the links to form a working ring and a protection ring in a ring topology, and establishing a plurality of working paths on the working ring and a plurality of protection paths on the protection ring corresponding to the plurality of working paths. In the network, one of the working paths spans across first and second nodes of the plurality of nodes for transmission of a signal in a first direction of the ring topology, and one of the protection paths spans across the first and second nodes for transmission of a signal in a second direction of the ring topology opposite to the first direction. The first and second nodes normally use the working path that spans across them. Responsive to a failure of the working path, the nodes use the protection path that spans across them, instead of the failed working path.
According to a second aspect, the present invention provides a communications network comprising a plurality of transmission links, and a plurality of nodes for interconnecting the links to form first and second working rings and first and second protection rings in a ring topology, and establishing a plurality of working paths on each of the working rings and a plurality of protection paths on each of the protection rings corresponding to the plurality of working paths. A first working path of the first working ring spans across first and second nodes for transmission of a signal in a first direction of the ring topology, and a second working path of the second working ring spans across the first and second nodes for transmission of a signal in a second direction of the ring topology opposite to the first direction. A first protection path on the first protection ring spans across the first and second nodes for transmission of a signal in the second direction of the ring topology, and a second protection path of the second protection ring spans across the first and second nodes for transmission of a signal in the first direction of the ring topology. The first and second nodes normally use the first and second working paths, respectively. Responsive to a failure of one of the first and second working paths, the first and second nodes use a corresponding one of the first and second protection paths, instead of the failed working path.
According to a third aspect, the present invention provides a communications network comprising a plurality of transmission links; and a plurality of nodes for interconnecting the links to form a working ring and a protection ring in a ring topology, and establishing a plurality of working paths on the working ring and a plurality of extra traffic paths on the protection ring. One of the working paths spans across first and second nodes for transmission of a signal in a first direction of the ring topology and one of the extra traffic paths spans across the first and second nodes for transmission of a low-priority signal in a second direction of the ring topology opposite to the first direction. The first and second nodes normally use the working path that spans across them. When a failure occurs in the working path, the extra traffic path between the nodes is cleared and a short-haul protection path is established for using it instead of the failed working path. If the short-haul protection path is not successfully established due to a further failure, other extra traffic paths are cleared and a long-haul protection path is established for using it instead of the failed working path.
According to a further aspect, the present invention provides a communications network in which first and second working paths are assigned a first network resource and first and second protection paths are assigned a second network resource. The first node normally uses the first network resource and the first working path for transmission of signals, and in response to a failure in the first ring, uses the second network resource and the second protection path, instead of the first network resource and the first working path. The second node normally uses the second network resource and the second working path for transmission of signals, and in response to a failure of the second ring, uses the first network resource and the first protection path, instead of the second network resource and the second working path.
According to a still further aspect, the present invention provides a communications network in which first and second working paths are assigned first and second network resources, respectively, and first and second protection paths are assigned the second and first network resources, respectively. The first node normally uses the first network resource and the first working path for transmission of signals and is responsive to a failure of the first ring to use the second protection path instead of the first working path. The second node normally uses the second network resource and the second working path for transmission of signals and is responsive to a failure of the second ring to use the first protection path instead of the second protection path.
According to a still further aspect, the present invention provides a network node for a ring topology network, the network having first and second working rings and first and second protection rings in a ring topology, and a plurality of working paths on each of the working rings and a plurality of protection paths on each of the protection rings corresponding to the plurality of working paths, the network node comprising a first demultiplexer for receiving a multiplex signal from one of the working paths for producing drop-off signals, a first multiplexer for multiplexing add-up signals onto the working path, a first path switch connected between the first demultiplexer and the first multiplexer, a second demultiplexer for receiving a multiplex signal from one of the protection paths for producing drop-off signals, a second multiplexer for multiplexing add-up signals onto the protection path, a second switch connected between the second demultiplexer and the second multiplexer, a transmit protection switch, a receive protection switch, and control circuitry for monitoring the working path and controlling the transmit protection switch so that one of the add-up signals is coupled to the first multiplexer when no failure is detected in the working path and coupled to the second multiplexer when a failure is detected in the working path, and controlling the receive protection switch so that one of the drop-off signals of the first multiplexer is received when no failure is detected in the working path and one of the drop-off signals of the second multiplexer is received when the failure is detected.